Method and apparatus for selecting proximity services group in wireless communication system

ABSTRACT

A method and apparatus for selecting a device-to-device (D2D) group in a wireless communication system is provided. A user equipment (UE) detects at least two D2D groups, and if the UE has no capability for supporting the at least two D2D groups, the UE selects one D2D group among the at least two D2D groups based on at least one of measurement results from the at least two D2D groups or group priority of each of the at least two D2D groups.

TECHNICAL FIELD

The present invention relates to wireless communications, and morespecifically, to a method and apparatus for selecting a proximityservices (ProSe) group in a wireless communication system.

BACKGROUND ART

Universal mobile telecommunications system (UMTS) is a 3^(rd) generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). A long-term evolution (LTE) of UMTS is under discussion by the3^(rd) generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3GPP LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Recently, there has been a surge of interest in supporting directdevice-to-device (D2D) communication. This new interest is motivated byseveral factors, including the popularity of proximity-based services,driven largely by social networking applications, and the crushing datademands on cellular spectrum, much of which is localized traffic, andthe under-utilization of uplink frequency bands. 3GPP is targeting theavailability of D2D communication in LTE rel-12 to enable LTE become acompetitive broadband communication technology for public safetynetworks, used by first responders. Due to the legacy issues and budgetconstraints, current public safety networks are still mainly based onobsolete 2G technologies while commercial networks are rapidly migratingto LTE. This evolution gap and the desire for enhanced services have ledto global attempts to upgrade existing public safety networks. Comparedto commercial networks, public safety networks have much more stringentservice requirements (e.g., reliability and security) and also requiredirect communication, especially when cellular coverage fails or is notavailable. This essential direct mode feature is currently missing inLTE.

From a technical perspective, exploiting the nature proximity ofcommunicating devices may provide multiple performance benefits. First,D2D user equipments (UEs) may enjoy high data rate and low end-to-enddelay due to the short-range direct communication. Second, it is moreresource-efficient for proximate UEs to communicate directly with eachother, versus routing through an evolved NodeB (eNB) and possibly thecore network. In particular, compared to normal downlink/uplink cellularcommunication, direct communication saves energy and improves radioresource utilization. Third, switching from an infrastructure path to adirect path offloads cellular traffic, alleviating congestion, and thusbenefitting other non-D2D UEs as well. Other benefits may be envisionedsuch as range extension via UE-to-UE relaying.

From an economic perspective, LTE D2D should create new businessopportunities, though its commercial applications are not the focus inLTE rel-12. For example, many social networking applications rely on theability to discover users that are in proximity, but the devicediscovery processes typically work in a non-autonomous manner. Usersfirst register their location information in a central server oncelaunching the application. The central server then distributes theregistered location information to other users using the application. Itwould be appealing to the service providers if device discovery can workautonomously without manual location registration. Other examplesinclude e-commerce, whereby private information need only be sharedlocally between two parties, and large file transfers, e.g., just-takenvideo clips shared amongst other nearby friends.

Thus far, use cases of 3GPP proximity services (ProSe) and correspondingarchitecture enhancements have been specified and studied. However, howthe UE selects one of detected ProSe groups is not clearly defined.Accordingly, a method for selecting a ProSe group is required.

SUMMARY OF INVENTION Technical Problem

The present invention provides a method and apparatus for selecting aproximity services (ProSe) group in a wireless communication system. Thepresent invention provides a method for selecting a ProSe group amongdetected ProSe groups if a user equipment (UE) has no capability forsupporting the detected ProSe groups.

Solution to Problem

In an aspect, a method for selecting, by a user equipment (UE), adevice-to-device (D2D) group in a wireless communication system isprovided. The method includes detecting at least two D2D groups, and ifthe UE has no capability for supporting the at least two D2D groups,selecting one D2D group among the at least two D2D groups based on atleast one of measurement results from the at least two D2D groups orgroup priority of each of the at least two D2D groups.

In another aspect, a user equipment (UE) in a wireless communicationsystem is provided. The UE includes a radio frequency (RF) unit fortransmitting or receiving a radio signal, and a processor coupled to theRF unit, and configured to detect at least two D2D groups, and if the UEhas no capability for supporting the at least two D2D groups, select oneD2D group among the at least two D2D groups based on at least one ofmeasurement results from the at least two D2D groups or group priorityof each of the at least two D2D groups.

Advantageous Effects of Invention

A ProSe group can be selected effectively.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical EPC.

FIG. 3 shows a block diagram of a user plane protocol stack and acontrol plane protocol stack of an LTE system.

FIG. 4 shows an example of a physical channel structure.

FIG. 5 and FIG. 6 show ProSe direct communication scenarios without arelay.

FIG. 7 shows reference architecture for ProSe.

FIG. 8 shows architecture for ProSe communications in group owner mode.

FIG. 9 shows ProSe-enabled UEs arranged in ProSe groups.

FIG. 10 shows an example of a method for selecting a D2D group accordingto an embodiment of the present invention.

FIG. 11 shows another example of a method for selecting a ProSe groupaccording to an embodiment of the present invention.

FIG. 12 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

MODE FOR THE INVENTION

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is evolved from IEEE 802.16e, and provides backwardcompatibility with a system based on the IEEE 802.16e. The UTRA is apart of a universal mobile telecommunication system (UMTS). 3^(rd)generation partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses theOFDMA in a downlink and uses the SC-FDMA in an uplink LTE-advanced(LTE-A) is an evolution of the LTE.

For clarity, the following description will focus on LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighboring eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical

EPC. Referring to FIG. 2, the eNB 20 may perform functions of selectionfor gateway 30, routing toward the gateway 30 during a radio resourcecontrol (RRC) activation, scheduling and transmitting of pagingmessages, scheduling and transmitting of broadcast channel (BCH)information, dynamic allocation of resources to the UEs 10 in both ULand DL, configuration and provisioning of eNB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE_IDLE state management,ciphering of the user plane, SAE bearer control, and ciphering andintegrity protection of NAS signaling.

FIG. 3 shows a block diagram of a user plane protocol stack and acontrol plane protocol stack of an LTE system. FIG. 3-(a) shows a blockdiagram of a user plane protocol stack of an LTE system, and FIG. 3-(b)shows a block diagram of a control plane protocol stack of an LTEsystem.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel. Data istransferred between the MAC layer and the PHY layer through thetransport channel. Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

FIG. 4 shows an example of a physical channel structure. A physicalchannel consists of a plurality of subframes in time domain and aplurality of subcarriers in frequency domain. One subframe consists of aplurality of symbols in the time domain. One subframe consists of aplurality of resource blocks (RBs). One RB consists of a plurality ofsymbols and a plurality of subcarriers. In addition, each subframe mayuse specific subcarriers of specific symbols of a corresponding subframefor a PDCCH. For example, a first symbol of the subframe may be used forthe PDCCH. The PDCCH carries dynamic allocated resources, such as aphysical resource block (PRB) and modulation and coding scheme (MCS). Atransmission time interval (TTI) which is a unit time for datatransmission may be equal to a length of one subframe. The length of onesubframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel. A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving an RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink The MTCH is apoint-to-multipoint downlink channel for transmitting traffic data fromthe network to the UE.

Uplink connections between logical channels and transport channelsinclude the

DCCH that can be mapped to the UL-SCH, the DTCH that can be mapped tothe UL-SCH and the CCCH that can be mapped to the UL-SCH. Downlinkconnections between logical channels and transport channels include theBCCH that can be mapped to the BCH or DL-SCH, the PCCH that can bemapped to the PCH, the DCCH that can be mapped to the DL-SCH, and theDTCH that can be mapped to the DL-SCH, the MCCH that can be mapped tothe MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom a higher layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, can be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

Referring to FIG. 3-(a), the RLC and MAC layers (terminated in the eNBon the network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARQ). ThePDCP layer (terminated in the eNB on the network side) may perform theuser plane functions such as header compression, integrity protection,and ciphering.

Referring to FIG. 3-(b), the RLC and MAC layers (terminated in the eNBon the network side) may perform the same functions for the controlplane. The RRC layer (terminated in the eNB on the network side) mayperform functions such as broadcasting, paging, RRC connectionmanagement, RB control, mobility functions, and UE measurement reportingand controlling. The NAS control protocol (terminated in the MME ofgateway on the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

An RRC state indicates whether an RRC layer of the UE is logicallyconnected to an

RRC layer of the E-UTRAN. The RRC state may be divided into twodifferent states such as an RRC connected state and an RRC idle state.When an RRC connection is established between the RRC layer of the UEand the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, andotherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has theRRC connection established with the E-UTRAN, the E-UTRAN may recognizethe existence of the UE in RRC_CONNECTED and may effectively control theUE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN,and a CN manages the UE in unit of a TA which is a larger area than acell. That is, only the existence of the UE in RRC_IDLE is recognized inunit of a large area, and the UE must transition to RRC_CONNECTED toreceive a typical mobile communication service such as voice or datacommunication.

In RRC_IDLE state, the UE may receive broadcasts of system informationand paging information while the UE specifies a discontinuous reception(DRX) configured by NAS, and the UE has been allocated an identification(ID) which uniquely identifies the UE in a tracking area and may performpublic land mobile network (PLMN) selection and cell re-selection. Also,in RRC_IDLE state, no RRC context is stored in the eNB.

In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and acontext in the E-UTRAN, such that transmitting and/or receiving datato/from the eNB becomes possible. Also, the UE can report channelquality information and feedback information to the eNB. InRRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs.Therefore, the network can transmit and/or receive data to/from UE, thenetwork can control mobility (handover and inter-radio accesstechnologies (RAT) cell change order to GSM EDGE radio access network(GERAN) with network assisted cell change (NACC)) of the UE, and thenetwork can perform cell measurements for a neighboring cell.

In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically,the UE monitors a paging signal at a specific paging occasion of everyUE specific paging DRX cycle. The paging occasion is a time intervalduring which a paging signal is transmitted. The UE has its own pagingoccasion.

A paging message is transmitted over all cells belonging to the sametracking area. If the UE moves from one TA to another TA, the UE willsend a tracking area update (TAU) message to the network to update itslocation.

When the user initially powers on the UE, the UE first searches for aproper cell and then remains in RRC IDLE in the cell. When there is aneed to establish an RRC connection, the UE which remains in RRC_IDLEestablishes the RRC connection with the RRC of the E-UTRAN through anRRC connection procedure and then may transition to RRC_CONNECTED. TheUE which remains in RRC_IDLE may need to establish the RRC connectionwith the E-UTRAN when uplink data transmission is necessary due to auser's call attempt or the like or when there is a need to transmit aresponse message upon receiving a paging message from the E-UTRAN.

It is known that different cause values may be mapped o the signaturesequence used to transmit messages between a UE and eNB and that eitherchannel quality indicator (CQI) or path loss and cause or message sizeare candidates for inclusion in the initial preamble.

When a UE wishes to access the network and determines a message to betransmitted, the message may be linked to a purpose and a cause valuemay be determined. The size of the ideal message may be also bedetermined by identifying all optional information and differentalternative sizes, such as by removing optional information, or analternative scheduling request message may be used.

The UE acquires necessary information for the transmission of thepreamble, UL interference, pilot transmit power and requiredsignal-to-noise ratio (SNR) for the preamble detection at the receiveror combinations thereof. This information must allow the calculation ofthe initial transmit power of the preamble. It is beneficial to transmitthe UL message in the vicinity of the preamble from a frequency point ofview in order to ensure that the same channel is used for thetransmission of the message.

The UE should take into account the UL interference and the UL path lossin order to ensure that the network receives the preamble with a minimumSNR. The UL interference can be determined only in the eNB, andtherefore, must be broadcast by the eNB and received by the UE prior tothe transmission of the preamble. The UL path loss can be considered tobe similar to the DL path loss and can be estimated by the UE from thereceived RX signal strength when the transmit power of some pilotsequence of the cell is known to the UE.

The required UL SNR for the detection of the preamble would typicallydepend on the eNB configuration, such as a number of Rx antennas andreceiver performance. There may be advantages to transmit the ratherstatic transmit power of the pilot and the necessary UL SNR separatelyfrom the varying UL interference and possibly the power offset requiredbetween the preamble and the message.

The initial transmission power of the preamble can be roughly calculatedaccording to the following formula:

Transmit power=TransmitPilot−RxPilot+ULInterference+Offset+SNRRequired

Therefore, any combination of SNRRequired, ULlnterference, TransmitPilotand

Offset can be broadcast. In principle, only one value must be broadcast.This is essentially in current UMTS systems, although the ULinterference in 3GPP LTE will mainly be neighboring cell interferencethat is probably more constant than in UMTS system.

The UE determines the initial UL transit power for the transmission ofthe preamble as explained above. The receiver in the eNB is able toestimate the absolute received power as well as the relative receivedpower compared to the interference in the cell. The eNB will consider apreamble detected if the received signal power compared to theinterference is above an eNB known threshold.

The UE performs power ramping in order to ensure that a UE can bedetected even if the initially estimated transmission power of thepreamble is not adequate. Another preamble will most likely betransmitted if no ACK or NACK is received by the UE before the nextrandom access attempt. The transmit power of the preamble can beincreased, and/or the preamble can be transmitted on a different ULfrequency in order to increase the probability of detection. Therefore,the actual transmit power of the preamble that will be detected does notnecessarily correspond to the initial transmit power of the preamble asinitially calculated by the UE.

The UE must determine the possible UL transport format. The transportformat, which may include MCS and a number of resource blocks thatshould be used by the UE, depends mainly on two parameters, specificallythe SNR at the eNB and the required size of the message to betransmitted.

In practice, a maximum UE message size, or payload, and a requiredminimum SNR correspond to each transport format. In UMTS, the UEdetermines before the transmission of the preamble whether a transportformat can be chosen for the transmission according to the estimatedinitial preamble transmit power, the required offset between preambleand the transport block, the maximum allowed or available UE transmitpower, a fixed offset and additional margin. The preamble in UMTS neednot contain any information regarding the transport format selected bythe EU since the network does not need to reserve time and frequencyresources and, therefore, the transport format is indicated togetherwith the transmitted message.

The eNB must be aware of the size of the message that the UE intends totransmit and the SNR achievable by the UE in order to select the correcttransport format upon reception of the preamble and then reserve thenecessary time and frequency resources. Therefore, the eNB cannotestimate the SNR achievable by the EU according to the received preamblebecause the UE transmit power compared to the maximum allowed orpossible UE transmit power is not known to the eNB, given that the UEwill most likely consider the measured path loss in the DL or someequivalent measure for the determination of the initial preambletransmission power.

The eNB could calculate a difference between the path loss estimated inthe DL compared and the path loss of the UL. However, this calculationis not possible if power ramping is used and the UE transmit power forthe preamble does not correspond to the initially calculated UE transmitpower. Furthermore, the precision of the actual UE transmit power andthe transmit power at which the UE is intended to transmit is very low.Therefore, it has been proposed to code the path loss or CQI estimationof the downlink and the message size or the cause value in the UL in thesignature.

Proximity Services (ProSe) are described. It may be refer to 3GPP TR23.703 V0.4.1 (2013-06). The ProSe may be a concept including adevice-to-device (D2D) communication. Hereinafter, the ProSe may be usedby being mixed with a device-to-device (D2D).

ProSe direct communication means a communication between two or more UEsin proximity that are ProSe-enabled, by means of user plane transmissionusing E-UTRA technology via a path not traversing any network node.ProSe-enabled UE means a UE that supports ProSe requirements andassociated procedures. Unless explicitly stated otherwise, aProSe-enabled UE refers both to a non-public safety UE and a publicsafety UE. ProSe-enabled public safety UE means a ProSe-enabled UE thatalso supports ProSe procedures and capabilities specific to publicsafety. ProSe-enabled non-public safety UE means a UE that supportsProSe procedures and but not capabilities specific to public safety.ProSe direct discovery means a procedure employed by a ProSe-enabled UEto discover other ProSe-enabled UEs in its vicinity by using only thecapabilities of the two UEs with 3GPP LTE rel-12 E-UTRA technology.EPC-level ProSe discovery means a process by which the EPC determinesthe proximity of two ProSe-enabled UEs and informs them of theirproximity.

When the registered public land mobile network (PLMN), ProSe directcommunication path and coverage status (in coverage or out of coverage)are considered, there are a number of different possible scenarios.Different combinations of direct data paths and in-coverage andout-of-coverage may be considered.

FIG. 5 and FIG. 6 show ProSe direct communication scenarios without arelay. FIG. 5-(a) shows a case that UE1 and UE2 are out of coverage.FIG. 5-(b) shows a case that UE1 is in coverage and in PLMN A, and UE2is out of coverage. FIG. 5-(c) shows a case that UE1 and UE2 are incoverage and in PLMN A, and UE1 and UE2 shares the same PLMN A and thesame cell. FIG. 5-(d) shows a case that UE1 and UE2 are in coverage andin the same PLMN A, but UE1 and UE2 are in different cells each other.FIG. 6-(a) shows a case that UE1 and UE2 are in coverage, but UE1 andUE2 are in different PLMNs (i.e., PLMN A/B) and different cells eachother. UE1 and UE2 are in both cells' coverage. FIG. 6-(b) shows a casethat UE1 and UE2 are in coverage, but UE1 and UE2 are in different PLMNs(i.e., PLMN A/B) and different cells each other. UE1 is in both cells'coverage and UE2 is in serving cell's coverage. FIG. 6-(c) shows a casethat UE1 and UE2 are in coverage, but UE1 and UE2 are in different PLMNs(i.e., PLMN A/B) and different cells each other. UE1 and UE2 are in itsown serving cell's coverage. In the description above, “in coverage andin PLMN A” means that the UE is camping on the cell of the PLMN A andunder the control of the PLMN A.

The ProSe direct communication scenarios described above may not coverall possible ProSe communication scenarios. Additional scenarios couldbe added. The ProSe direct communication scenarios described above areall applicable in cases of network sharing. It is for further studywhether the scenarios described above could be enhanced or additionalfigures should to be added to further clarify the scenarios in relationto network sharing. It is for further study whether ProSe directcommunication scenarios described above should be enhanced or new ProSecommunication scenarios should be added for group communication. It isfor further study whether ProSe direct communication scenarios describedabove should be enhanced or new ProSe communication scenarios should beadded to cover further roaming cases.

Two different modes for ProSe direct communication one-to-one may besupported.

-   -   Network independent direct communication: This mode of operation        for ProSe direct communication does not require any network        assistance to authorize the connection and communication is        performed by using only functionality and information local to        the UE. This mode is applicable only to pre-authorized        ProSe-enabled public safety UEs, regardless of whether the UEs        are served by E-UTRAN or not.    -   Network authorized direct communication: This mode of operation        for ProSe direct communication always requires network        assistance and may also be applicable when only one UE is        “served by E-UTRAN” for public safety UEs. For non-public safety        UEs both UEs must be “served by E-UTRAN”.

FIG. 7 shows reference architecture for ProSe. Referring to FIG. 7, thereference architecture for ProSe includes E-UTRAN, EPC, plurality of UEshaving ProSe applications, ProSe application server, and ProSe function.The EPC represents the E-UTRAN core network architecture. The EPC mayinclude entities such as MME, S-GW, P-GW, policy and charging rulesfunction (PCRF), home subscriber server (HSS), etc. The ProSeapplication servers are users of the ProSe capability for building theapplication functionality. In the public safety cases, they may bespecific agencies (PSAP), or in the commercial cases social media. Theseapplications may be defined outside the 3GPP architecture but there maybe reference points towards 3GPP entities. The application server cancommunicate towards an application in the UE. Applications in the UE usethe ProSe capability for building the application functionality. Examplemay be for communication between members of public safety groups or forsocial media application that requests to find buddies in proximity.

The ProSe Function in the network (as part of EPS) defined by 3GPP has areference point towards the ProSe application server, towards the EPCand the UE. The functionality may include at least one of followings.But the functionality may not be restricted to the followings.

-   -   Interworking via a reference point towards the 3rd party        applications    -   Authorization and configuration of the UE for discovery and        direct communication    -   Enable the functionality of the EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of EPC, e.g.,        offline charging)

Reference points/interfaces in the reference architecture for ProSe aredescribed.

-   -   PC1: It is the reference point between the ProSe application in        the UE and in the ProSe application server. It is used to define        application level signalling requirements.    -   PC2: It is the reference point between the ProSe application        server and the ProSe function. It is used to define the        interaction between ProSe application server and ProSe        functionality provided by the 3GPP EPS via ProSe function. One        example may be for application data updates for a ProSe database        in the ProSe function. Another example may be data for use by        ProSe application server in interworking between 3GPP        functionality and application data, e.g., name translation.    -   PC3: It is the reference point between the UE and ProSe        function. It is used to define the interaction between UE and        ProSe function. An example may be to use for configuration for        ProSe discovery and communication.    -   PC4: It is the reference point between the EPC and ProSe        function. It is used to define the interaction between EPC and        ProSe function. Possible use cases may be when setting up a        one-to-one communication path between UEs or when validating        ProSe services (authorization) for session management or        mobility management in real time. Depending on the function        needed, PC4 may terminate in different EPC entities and may        reuse existing interfaces.    -   PC5: It is the reference point between UE to UE used for control        and user plane for discovery and communication, for relay and        one-to-one communication (between UEs directly and between UEs        over LTE-Uu).    -   PC6: This reference point may be used for functions such as        ProSe discovery between users subscribed to different PLMNs.    -   SGi: In addition to the relevant functions via SGi, it may be        used for application data and application level control        information exchange.

Applicability of the above interfaces/reference points may be dependenton solutions developed for ProSe.

For ProSe communication, various solutions have been discussed. One ofthe solutions for the ProSe communication, ProSe communications in groupowner mode has been discussed. The ProSe group owner mode may address“ProSe one-to-many communications”, which is one of key issues of ProSecommunication. The ProSe one-to-many communications refers to ProSegroup communication and ProSe broadcast communication. One-to-manycommunications may also work without prior discovery. It is designed towork in or out of network coverage. Further, the ProSe group owner modemay be used whenever there is a possibility for stable group ownership,such as the relay use cases (i.e., UE-to-UE relay and UE-to-networkrelay). In that sense, the ProSe group owner mode may address “relay forpublic safety ProSe” key issue. Relays are specific to public safety usecases. The relays can be used for both ProSe communication one-to-oneand one-to-many. Further, the ProSe group owner mode may also be usedfor ProSe one-to-one communication, and accordingly, the Prose groupowner mode may address “ProSe direct communication one-to-one” keyissue.

FIG. 8 shows architecture for ProSe communications in group owner mode.Referring to FIG. 8, architecture for ProSe communications in groupowner mode includes a ProSe group, and the ProSe group includes aplurality of UEs, i.e., UE1, UE2, and UE3. UE3 acts as a group owner(GO) of the ProSe group. A reference point PC5 has been enhancedcompared with FIG. 7. In FIG. 8, reference point PC5 is the “lowerlayer” (ProSe) reference point between a UE and a UE acting as a ProSeGO. It provides basic IP connectivity between the two UEs.

Before direct communication can be established between two or multipleProSe-enabled UEs, these UEs need to become members of the same ProSeGroup. Essentially, a ProSe group is a private IP network composed byone or more member UEs that can securely communicate with any IP-basedapplication. Each ProSe group is characterised by a locally uniqueidentity. The procedure with which a new ProSe group is created iscalled group formation procedure.

One of the UEs in a ProSe group plays the role of the GO (UE3 in FIG.8), i.e., implements special functionality that facilitates the groupformation and operation. The GO is similar to a wireless access router.That is, it announces the group (e.g., by broadcasting a certain groupidentity) and admits or rejects new UEs that request to become groupmembers. It also authenticates new group members and provides them withIP configuration data. The GO creates the group identity with apseudo-random fashion so that the group identity can be assumed locallyunique, i.e., unique across all other groups that operate in the samearea. The GO is within the transmission range of all ProSe groupmembers, however, the ordinary (non-GO) ProSe group members need not bewithin transmission range of each other.

There is a clear ProSe group formation procedure whereby individualmembers join the ProSe group by performing mutual authentication withthe GO, or each ProSe group member is assigned an IP address/prefix bythe GO, or the ProSe group members form a distinct IP subnet.

All traffic exchanged within the ProSe group is forwarded via the GO.The GO behaves also as a communication bridge, i.e., it receives alltransmissions from the group members and (if necessary) forwards thetransmissions to other group members (e.g., based on the L2 or L3destination address). Multicast traffic (i.e., traffic destined to someor all ProSe group members) sent by an ordinary ProSe group member isdelivered in unicast mode to the GO, which subsequently distributes itto all ProSe group members. The distribution from the GO can be ineither unicast or multicast mode (e.g., depending on the number of ProSegroup members). The GO may be in position to perform a centralised radioresource control from the GO. The GO may be able to provide some QoSsupport.

Strong security may be the salient feature of the architecture for ProSecommunications in group owner. The GO may be in position to authenticateeach UE individually and generate security material during the groupformation procedure.

ProSe one-to-many communications in group owner mode are IP-based. IPpackets are encapsulated within layer-2 frames. As a minimum, thelayer-2 frame header consists of the following fields.

-   -   Destination Layer-2 ID: this identifier can take the form of        either an individual (unicast) or a group (multicast)        identifier. Multicast identifiers are used when the data        distribution from the GO to the group members is in multicast        mode.    -   Source Layer-2 ID: this identifier is always set to the        individual (unicast) identifier of the sender's device.

UEs engaging in joining the same ProSe group learn their respectiveLayer-2 IDs during the group formation procedure. Multicast Layer-2 IDs(used only when the GO relies on multicast distribution to other ProSegroup members) are assigned using application-layer signaling.

A ProSe group can be formed either autonomously by one or more UEswithout any network involvement or with network assistance. Theautonomous ProSe group formation procedure is particularly useful inscenarios where a ProSe group needs to be formed outside the networkcoverage, e.g., to enable direct communication between public safety UEsthat need to handle an emergency situation in an isolated area or wherenetwork coverage is not available.

FIG. 9 shows ProSe-enabled UEs arranged in ProSe groups. Referring toFIG. 9, there are four ProSe groups. ProSe group A includes UE1 and UE2,and UE1 act as a GO in the ProSe group A. ProSe group B includes UE3,UE4, and UE5, and UE4 act as a GO in the ProSe group B. ProSe group Cincludes UE2, UE3, and UE6, and UE6 act as a GO in the ProSe group C.ProSe group D includes only UE7, which acts as a GO in the ProSe groupD.

An autonomous ProSe group may be created with one of the following ways:

-   -   A ProSe-enabled UE may create a ProSe group by autonomously        becoming a GO (refer ProSe group D in FIG. 9). This UE creates a        new group identity and advertises the existence of the ProSe        group (e.g., by broadcasting the group identity on designated        radio channel(s)) and serves requests from other ProSe-enabled        UEs that want to become group members. In this case, the ProSe        group starts as a single-member group (the GO is the only        initial member).    -   A ProSe-enabled UE may start a ProSe group after it discovers        (e.g., by using ProSe direct discovery) another ProSe-enabled UE        in close proximity. These two UEs negotiate the GO role, i.e.,        one of them is elected to function as GO. In this case, the        ProSe group starts as a two-member group.

An autonomous ProSe group can be expanded when other UEs request to jointhe group or when other UEs are invited to join the group. Theinvitation to the group is useful when two UEs need to directlycommunicate, but they are not members of a common ProSe group. Referringto FIG. 9, for example, if UE5 in the ProSe group B wants to establishdirect communication with UE8, which is not a member of ProSe group B,UE5 may send an invitation request message to UE8 and trigger it to joinProSe group B.

Only the GO can accept new members to the group, thus all join requestsneed to be sent to the GO. The GO can be discovered by its broadcasttransmissions. When the GO accepts a new ProSe-enabled UE to join thegroup, it provides to this UE the necessary security information (apre-shared key) for securing all further communications with the GO.

When two ProSe-enabled UEs need to establish direct communication, theyneed first to create a new ProSe group. One of these UEs will beassigned the role of GO and, subsequent to mutual authentication, willprovide IP configuration information to the other UE. If necessary, thegroup may be kept closed by rejecting other UEs to join this group.

An autonomous ProSe group can be created by using E-UTRA Rel-12 radiotechnology.

The ProSe group may be also considered as a cluster. The GO of the ProSegroup may be also considered as a cluster head (CH). Further,ProSe-enabled UEs may be called just UEs in the description below.

A ProSe group may be initially created according to the followingprocedure.

1. UE1 broadcasts a group invitation with reference signals. The groupinvitation may include a group ID and UE ID of the UE1. The groupidentity may be assigned to the UE1, UE2 and other UEs by the network,while UEs are in network coverage.

2. The UE2 and other UEs listen to the group invitation and thendetermine D2D timing based on the reference signals broadcast by theUE1, if they have the same group ID as the group ID included in thegroup invitation.

3. The UE2 and other UEs transmit a joining request to the UE1, if theyhave the same group ID as the group ID included in the group invitation.The joining request may include the group ID and UE ID of each UE.

4. The UE1 transmits a joining response to the UE2 and other UEs.Finally, the UE1 is considered as a temporary ProSe GO, and other UEsare considered as ProSe group members. The ProSe group members maydetermine D2D timing based on reference signal transmitted by the GO.The GO also relays D2D traffic between a group member and the E-UTRANserving the GO.

Hereinafter, a method for selecting a ProSe group according to anembodiment of the present invention is described. In a certainsituation, the UE may detect multiple ProSe groups, and accordingly, theUE should select one ProSe group among the detected ProSe groups. Whenthe UE does not join any ProSe group, the UE may select one ProSe groupamong detected ProSe groups and join the selected ProSe group.Alternatively, while the UE joins one ProSe group, the UE may selectanother ProSe group among detected ProSe group and join the selectedProSe group.

FIG. 10 shows an example of a method for selecting a D2D group accordingto an embodiment of the present invention.

In step S100, the UE detects at least two D2D groups. The UE may detectthe at least two D2D groups based on reference signals from the at leasttwo D2D groups. The UE may measure the reference signals received fromthe at least two D2D groups. Further, the UE may receive IDs for the atleast two D2D groups from a network. Further, the UE may receive thegroup priority of each of the at least two D2D groups from a network orfrom the at least two D2D groups.

In step S110, if the UE has no capability for supporting the at leasttwo D2D groups, the UE selects one D2D group among the at least two D2Dgroups based on at least one of measurement results from the at leasttwo D2D groups or group priority of each of the at least two D2D groups.The selected one D2D group may have a higher priority than other D2Dgroups among the at least two D2D groups. Or, the selected one D2D groupmay have a better measure signal quality than other D2D groups among theat least two D2D groups. After selecting the one D2D group, the UE maytransmit a joining request to the selected one D2D group. The UE mayleave the previously joined ProSe group.

FIG. 11 shows another example of a method for selecting a ProSe groupaccording to an embodiment of the present invention. It is assumed thatthe UE1 and UE3 serve as a GO of the first ProSe group and the secondProSe group, respectively.

In step S200, the network may provide IDs of two ProSe group to the UE2,while the UE2 is in network coverage. The network may also provide agroup priority for each ProSe group to the UE2. For instance, a ProSegroup for public safety may have a higher group priority than a ProSegroup for commercial purpose.

In step S210, the UE2 may detect both the first ProSe group and secondProSe group in a short interval, e.g., by listening to reference signalsor group invitation messages from both the UE1 and UE3. The UE2 mayacquire a group priority for the corresponding ProSe group via the groupinvitation message.

In step S220, if the UE2 has ProSe group IDs corresponding to both thefirst ProSe group and second ProSe group, and if the UE2 has nocapability of supporting two ProSe groups in parallel, the UE2 selectsone of detected ProSe groups, e.g., based on measurement results ofreference signals from both the UE1 and UE3 or the group priority foreach ProSe group. The UE2 may select the ProSe group that has a highergroup priority and/or a better measured signal quality compared toanother ProSe group. If the UE2 has capability of supporting two ProSegroups in parallel, the UE2 may select both detected ProSe groups andjoin both ProSe groups.

In step S230, the UE2 transmits a joining request to the GO of theselected ProSe group (i.e., UE1 and/or UE3), following the ProSe groupinitialization procedure described above.

FIG. 12 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

A network 800 may include a processor 810, a memory 820 and a radiofrequency (RF) unit 830. The processor 810 may be configured toimplement proposed functions, procedures and/or methods described inthis description. Layers of the radio interface protocol may beimplemented in the processor 810. The memory 820 is operatively coupledwith the processor 810 and stores a variety of information to operatethe processor 810. The RF unit 830 is operatively coupled with theprocessor 810, and transmits and/or receives a radio signal.

A UE 900 may include a processor 910, a memory 920 and a RF unit 930.The processor 910 may be configured to implement proposed functions,procedures and/or methods described in this description. Layers of theradio interface protocol may be implemented in the processor 910. Thememory 920 is operatively coupled with the processor 910 and stores avariety of information to operate the processor 910. The RF unit 930 isoperatively coupled with the processor 910, and transmits and/orreceives a radio signal.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

1. A method for selecting, by a user equipment (UE), a device-to-device(D2D) group in a wireless communication system, the method comprising:detecting at least two D2D groups; and if the UE has no capability forsupporting the at least two D2D groups, selecting one D2D group amongthe at least two D2D groups based on at least one of measurement resultsfrom the at least two D2D groups or group priority of each of the atleast two D2D groups.
 2. The method of claim 1, wherein the selected oneD2D group has a higher priority than other D2D groups among the at leasttwo D2D groups.
 3. The method of claim 1, wherein the selected one D2Dgroup has a better measure signal quality than other D2D groups amongthe at least two D2D groups.
 4. The method of claim 1, furthercomprising: receiving identities (IDs) for the at least two D2D groupsfrom a network.
 5. The method of claim 1, further comprising: receivingthe group priority of each of the at least two D2D groups from anetwork.
 6. The method of claim 1, further comprising: receiving thegroup priority of each of the at least two D2D groups from the at leasttwo D2D groups.
 7. The method of claim 6, wherein the group priority ofeach of the at least two D2D groups is received via a group invitationmessage.
 8. The method of claim 1, wherein the measurement results fromthe at least two D2D groups are based on reference signals received fromthe at least two D2D groups.
 9. The method of claim 1, furthercomprising: transmitting a joining request to the selected one D2Dgroup.
 10. The method of claim 9, further comprising: leaving a D2Dgroup where the UE joined previously.
 11. A user equipment (UE) in awireless communication system, the UE comprising: a radio frequency (RF)unit for transmitting or receiving a radio signal; and a processorcoupled to the RF unit, and configured to: detect at least two D2Dgroups; and if the UE has no capability for supporting the at least twoD2D groups, select one D2D group among the at least two D2D groups basedon at least one of measurement results from the at least two D2D groupsor group priority of each of the at least two D2D groups.
 12. The UE ofclaim 11, wherein the selected one D2D group has a higher priority thanother D2D groups among the at least two D2D groups.
 13. The UE of claim11, wherein the selected one D2D group has a better measure signalquality than other D2D groups among the at least two D2D groups.
 14. TheUE of claim 11, wherein the processor is further configured to: receiveidentities (IDs) for the at least two D2D groups from a network.
 15. TheUE of claim 11, wherein the measurement results from the at least twoD2D groups are based on reference signals received from the at least twoD2D groups.